1. Field of the Invention
The invention relates to a process for the direct synthesis of methylchlorosilanes using a contact composition comprising sodium and/or potassium.
2. Background Art
Processes for preparing methylchlorosilanes by reaction of silicon with chloromethane in the Müller-Rochow direct synthesis in the presence of suitable catalysts and catalyst combinations are already known. For example, this is described in CATALYZED DIRECT REACTIONS OF SILICON, K. M. Lewis, D. G. Rethwisch; Elsevier 1993.
In the direct synthesis of methylchlorosilanes, metallic silicon is reacted with chloromethane in the presence of various catalysts, and optionally promoters, with the target product being dimethyldichlorosilane. The mixture of silicon, catalysts and promoters is referred to as the “contact composition”. As a worldwide total of over 1,500,000 metric tons of dimethyldichlorosilane is produced annually, very small improvements in the production process, for example an increase in the dimethyldichlorosilane selectivity, an increase in the dimethyldichlorosilane-specific space-time yield, or an increase in the specific raw materials yield, therefore have a large economic effect.
DE 3841417 A1 describes a “silicon atomized by means of inert gas” having an Na content of 0–0.5% and a K content of 0–0.5%. These figures are very imprecise, especially in the light of SILICON FOR THE CHEMICAL INDUSTRY, Geiranger, Norway; Jun. 16–18, 1992; pages 11–23; “Impurity Distribution in Silicon”, A. Schei, H. Rong, A. G. Forwald, it is clearly shown in Table 1 and FIG. 1 that in the production of refined, metallurgical silicon, as is used in the direct synthesis, Na and K impurities mainly leave the process via the offgases/dust and are only found to a very small extent in the silicon. A value of <5 ppm is explicitly indicated for Na in silicon. In Harry Morten Rong; SILICON FOR THE DIRECT PROCESS TO METHYLCHLOROSILANES; Doctorate Thesis, 1992, Norway, it is furthermore stated on pages 55–56 that the alkali metals are normally taken off with the offgas in the production of silicon.
European published application EP 470020 A1 states that 0.05–2% by weight of Li, Na, K, Rb or Cs is present in contact compositions, while U.S. Pat. No. 4,661,613 A describes the use of 0.05–4% by weight of Cs in the direct synthesis, with up to 90% by weight of the claimed amount of Cs being able to be replaced by Li, Na, K, Rb. The theoretical Na, K content is thus in the range from 0 to 3.6%. In the concrete examples, cesium is used. WO 2004/063206 describes a contact composition to which from 0.01 to 2% by weight of cesium, potassium or rubidium is added and which forms little carbon. In the actual examples, cesium was added.